1. Field of the Invention
The invention is to a high power, high frequency toroidal inductor with the core in segments with centrally located coils that produces lower losses, operates cooler, is more compact and lighter in weight.
2. Description of Related Art
A typical inductor consists of a coil, that creates flux, a magnetic core, that directs flux, and an air gap, that stores magnetic energy. The air gap is made of two flat faces of iron within the magnetic core.
Prior art magnet wires have limited use in making coil loops that carry large currents. The problem is exacerbated by the difficulty in transferring the heat produced by the wire into the air. Above 200 amps the design of the inductor is dictated more by thermal issues than by inductance issues.
Large currents can be carried by rectangular sectioned copper conductors that are pre-insulated by coating. Such insulated conductors are called magnet wire. When the magnet wire is wound around a toroidal core, it has to be bent 90 degrees four times in order to make one complete loop. When the magnet wire is thicker than 0.125 inch, it becomes difficult to sharply bend magnet wire without damaging the insulation coating. Bending a thick magnet wire with sharp radius will bend the insulation coating also. During bending, the inner insulation coating layer gets compressed, while the outer insulation layer gets stretched. The stretching and compression introduce very large stresses in the insulating layer. The polymeric materials, usually used, are not as strong as metals and their ability to stretch or compress is extremely limited and their tensile strengths are extremely low when compared to that of copper. While the copper conductor can deform and bend sharply, the insulation coating cannot stretch in the same way without cracking. A sharp bending will stretch the insulation coatings so severely that they crack.
Hernandez et al, U.S. Pat. No. 5,396,212, issued 7 Mar. 1995, addresses wire bending and shapes that produce the best electrical characteristics. F. Benke, U.S. Pat. No. 3,633,272, issued 11 Jan. 1972, addresses conductor thickness and width for insulation. Abe et al, U.S. Pat. No. 6,531,946, issued 11 Mar. 2003, addresses high frequency effect and the effect of various gaps in toroidal cores.
When the coils are energized, magnetic flux lines are concentrated through the cores. Most of such flux lines flow through iron, and whenever there is an air gap, they jump across the air gaps, perpendicular to the flat faces of iron that form the air gap. The air gap in a typical inductor can be as small as 0.010 inch or as large as 0.062 inch. Large air gapped inductors are not preferred as they emit significantly more fringe fields that generate harmful electromagnetic interference and heat up any nearby copper conductors.
Any part of flux lines where relative permeability equals 1 is called an “air gap.” Air gaps can be of two types. One, that is usually called the magnetic gap, is formed by flat iron faces that are narrow. Another, usually called fringe gap, is formed by curved iron faces with flux lines that extend outwardly perpendicular from one iron section, bend around an iron-to-iron gap, and extend perpendicular back into the adjacent iron section. A small portion of flux, called fringing flux, emanates from one iron face and jumps by way of the fringe gap into the nearest iron face.
Some of the fringe flux lines may penetrate a copper conductor in their path. Such flux lines, when changing rapidly, create a voltage that induces eddy currents in the copper in accordance with the Lenz law. These eddy currents create so-called fringe losses within the conductor. The fringe loss is especially severe at higher frequencies of 1 kHz and above. Modem pulse-width-modulated power converters produce such flux levels at higher frequencies. The fringe loss increases with the square of the frequency and flux density.
Magnetic gaps are introduced into the inductor by slitting the toroidal core. Some inductors used a single air-gap, M. DeGraff, U.S. Pat. No. 6,492,893, issued 10 Dec. 2002 and R. Chu, U.S. Pat. No. 6,762,666, issued 13 Jul. 2004, being examples. High power inductors require large amounts of energy to be stored. When large amounts of energy are stored in a single magnetic gap, it is found that significantly more energy dissipation occurs. One approach to reduce eddy loss breaks up a magnetic gap into several small air gaps. This can reduce the amount of energy that is dissipated. Multiple air-gapped toroids are proposed by Aldridge et al, U.S. Pat. No. 4,199,744, issued 22 Apr. 1980 and R. Charles, U.S. Pat. No. 5,165,162, issued 24 Nov. 1992.
Even though multiple air gaps can be used to reduce heat dissipation, prior art inductors made no deliberate attempt to prevent fringing flux lines from penetrating the coil windings.
A second approach relies on using Litz wire to self-cancel the eddy currents, a described in W. T. McLayman, U.S. Pat. No. 4,975,672, issued 4 Dec. 1990. The air gap made of two flat faces of iron is also exemplified by W. T. McLayman. Litz wire capable of carrying hundreds of amps is very large. Such large Litz wire is heavy, voluminous, costly and has poor life.
Bending large Litz diameter wires over sharp bending angles or small bend radius greatly increases bending stress in the insulation coating on the wire. After sustained operation over a long time, excessive stress damages the insulation coating. Even in a simple AWG2 magnet wire (with diameter of approximately 0.5 in.), if bent at 90 degrees, the insulation will crack and fail. One objective is to avoid this life-degradation, cost and weight penalties.
Because the Litz wire's copper fill factor is very low, it occupies more space than conventional copper conductors, thereby increasing the overall inductor volume. Copper fill factor is defined as the ratio of copper area and the geometrical winding window of the inductor. Larger diameter Litz wires increase the size of the coil. It also greatly increases the volume of the inductor since the wire occupies more space around it. The net result is that the Litz wire-based high power inductors tend to be extremely heavy and large.